addressed topresidentobamafieldtested positive for ricin poison. the day after a letter was also sent to senator roger wicker also tested positive for the same substance. >>> we begin with new information about the deadly bombing at the boston marathon. as the president and first lady prepare to travel to boston on thursday. after much speculation this afternoon, the fbi has releasted a statement. they say, quote, contrary to widespread reporting, there have been no arrests made in connection with the boston marathon attack. we do know investigators have identified solid leads from video evidence, including images of a person or persons carrying and dropping bags at the bombing scene. now, these pictures from whdh show a bag near a trash can at the blast site. just one part of the key evidence that investigators are pursuing to try to find the perpetrators. right now, there are conflicting reports as to how close they are to identifying any individuals. we expect more at an fbi briefing within the next hour. right now i want to go straight to nbc news justice correspondent, pete willi